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Hey guys (and gals)
Living in an historic home I have not wanted to put solar collectors on the roof to heat water. However, I have been thinking of all that heat in my attic. I live in Sacramento, ca and the attic fan, set at 102 degrees, runs at least 7 or 8 months of the year.

I wonder if I looped PEX pipe inside the roof rafters and put a layer of rigid foam insulation on the inside of the rafters i might be able to heat much of my hot water for almost free, at least in the daytime for those months.

Does this seem like a good idea? Whould I have to worry about condensation? What do you all think. What other problems do you all see as possible.

Sounds cool but I would use copper because PEX to ambient air is not a good heat exchange. Dont think you shoul insulate it though. Plus if you install a metal roof that could increase your conductivity if you could made it contact the copper. There is a Water heater that has a heat exchanger in it to increse its efficiency. But the installation is hard for me to explain in text and time consuming and boring. Anyway The radiated water would maintain the water in the tank and the water heater would only fire during recovery demand and that would still be assisted by your exchanger so you have comfort and efficiency. That manufacturer is A.O. Smith. They are $$$$ though And A.O. Smith is first in quality fo many years and make water heaters for many other manufacturers.

I think you should look into a taqnkless water heater intead. Sacramento gets cold in the winter. What then? Check out Rheem or Takagi for tankless. Go natural gas or propane if you can. Electric is just more expensive in the long run. Process of elimanation. Good luck.